Wireless communication is growing increasingly popular in all areas of business and personal use. Technology areas that use wireless communication include telecommunication, networks, Internet, and audio and video media. Furthermore, much of the wireless data is presented in a digital format, which in itself provides opportunities for improved processing. Resultantly, a constant need exists to improve performance and fidelity of wireless communication systems.
Referring now to prior art FIG. 1A, an illustration of multipath signal propagation occurring in wireless communication is shown. More specifically, the wireless communication occurs between two devices, e.g., a station 104 and a portable device 102. Station 104 can be a base station for a cell phone, while portable device 102 can be a mobile phone. Alternatively, station 104 can be an audio/video broadcaster that transmits digitally encoded data that is received by a television or a personal computer (PC) 102. The wireless communication system and the devices can span a wide variety of media and applications.
The problem of multipath interference in wireless communication is illustrated in prior art FIG. 1A. A transmitted signal can follow multiple paths, or multipaths, to arrive at a receiver. For example, one signal path, e.g., main 106a, transmits directly to a receiver 102 without any interference. However, due to natural and man-made obstructions, such as building 108, hill 110, and surface 112, almost duplicate versions of the original signal arrive at receiver 102 with slight variations in phase, amplitude, and angle of arrival at receiver 102. These multipaths can cause interference and destructive interference between each other.
Referring now to prior art FIG. 1B, a graph of signal performance with a specific interference referred to as fading is shown. Graph 100b has a spatial X-Y plane defined by axis X 124 and axis Y 126. The vertical axis represents amplitude 122 of a signal that can be received at different spatial X, Y locations. Plane 134 defines an approximate amplitude of a transmitted signal in relation to the spatial location of a receiver, e.g., an antennae. Areas of plane 134 having an attenuated amplitude 136 is indicative of Rayleigh fading of the signal. Rayleigh fading is characterized by spatially repeating deep fading areas that severely effects wireless communication performance. If an antennae is positioned in one or more of these pockets 136 of Rayleigh fading, then reception quality of a wireless signal can be significantly compromised. Multipath fading occurs when a receiver receives not only the direct signal from a transmitter, but also reflected signals that differ from the direct signal in amplitude, phase, and/or angle of arrival, e.g., multipath B 106b, multipath C 106c, and multipath D 106d of prior art FIG. 1A.
Multipath fading is directly related to the environment in which the receiver is working. For example, the Institute of Electrical and Electronic Engineers (IEEE) standard 802.11 for wireless local area network (LAN) employs frequencies of 2.4 gigahertz (GHz). However, at this frequency moving human bodies affect the multipath fading, sometimes to a greater extent than building construction. In fact, much research has shown that at 2.4 GHz, Rayleigh type fading occurs. Consequently, a need arises to overcome the Rayleigh fading that degrades wireless communication performance. A more specific need arises to solve multipath fading in applications, such as wireless LANs, that have a high sensitivity to this performance degradation.
One method of avoiding these deep fading areas, is to use antenna diversity at a receiver. Antenna diversity simply uses two antennas that are spatially or polarity separated such that only one antenna would be in a deep fade area. The problem with this solution is that the user's receiver in typical IEEE 802.11 application is usually implemented on a peripheral component (PC) card, which does not allow for a large enough antenna separation to substantially reduce the Rayleigh fading. Additionally, even if antennas with polarity diversity are used, they do not always provide adequate escape from Rayleigh fading. Consequently, a need arises for a method to avoid Rayleigh type fading when antennae diversity is ineffective or unsuitable.
Conventional methods have provided performance indicators, such as signal strength or SNR ratios, to indicate the reception quality of the radio. However, these methods and indicators are primarily for analog systems, and are not optimum for digital systems. Thus, their interpretation of good reception or a good signal is not necessarily applicable to that for a digital signal.
Referring now to prior art FIG. 1C, a graph of exemplary data and noise signals is shown. Graph 100c has an abscissa of time 124 and an ordinate of amplitude 120. Three cases of data/noise signal combinations are shown for illustrative purposes. Case 1 121 shows just a data signal 121d with no indication of noise amplitude. If a conventional performance indicator only provides the strength of a data signal, as shown in Case 1 121, then it can be misleading. While data signal may have acceptable amplitude, if a noise signal has sufficient amplitude, it may significantly corrupt the data signal. In contrast, Case 2 122 shows a data signal amplitude 122d and a noise signal amplitude 122n. By using the conventional signal-to-noise (SNR) ratio, a relative strength of the data signal with respect to the noise signal can be obtained. However, as shown by case 3 123, signals with substantially different absolute amplitudes can appear to be equal (e.g., case 2 122 and case 3 123) based only on their ratio of signal to noise. Based on limitations of conventional indicators, they are unsuitable, in general, to evaluate the accuracy and reception quality of a digital signal. Consequently, a need arises for a performance indicator that is more applicable to digital data.
Furthermore, the solution that eliminates or alleviates Rayleigh type fading and provides indication of good digital signal quality should be intuitive and simple to use, so as to promote implementation and compatibility. That is, the solution should not require the user to have apriori radio, computer, or any other technical knowledge to operate or understand it. In addition, the solution to Rayleigh fading and the indicator digital signal performance should not require significant interaction of the hosting device such that system performance is hampered.
In summary, a need exists to improve performance and fidelity of wireless communication. A need also arises to overcome the Rayleigh fading that degrades wireless communication performance. A more specific need arises to solve multipath fading in applications, such as wireless LANs, that have a high sensitivity to this performance degradation. Another need arises for a method to avoid Rayleigh type fading when antennae diversity is ineffective or unsuitable. Based on the limitations of conventional indicators, a need arises for a performance indicator that is more applicable to digital data. Furthermore, the solution that eliminates or alleviates Rayleigh type fading and provides indication of good digital signal quality should be intuitive and simple so as to promote implementation and compatibility. That is, the solution should not require the user to have apriori radio, computer, or any other technical knowledge to operate or understand it. In addition, the solution to Rayleigh fading and the indicator of digital signal performance should not excessively burden the hosting device such that system performance is hampered.